1. Field of the invention
The present invention relates to the field of semiconductor circuits, and more specifically, to the field of BiCMOS Output Drivers.
2. Description of Related Art
Output Drivers are well-known in the art. Output Drivers are used to drive a predetermined logic signal (i.e. logical high or logical low) out across a signal line to other chips or to other circuits on chip. Output Drivers "drive" the logical signal to other circuits by increasing (amplifying) the current of the signal supplied to the driver. It is important that the output voltage level of an Output Driver meet specified data sheet requirements. For example, TTL level Output Drivers must supply a voltage level of greater than 2.4 volts for a logical high signal and supply a voltage level of less than 0.4 volts for a logical low signal. It is also important that an Output Driver be able to meet data sheet voltage level requirements over a wide range of operating temperatures, typically between -55.degree. C. to 125.degree. C. The performance of an Output Driver can be approximated by the drivers ratio of Drive Capability/Input Capacitance. The larger the ratio, the better performing the Output Driver.
FIG. 1 shows an all NMOS Output Driver 100. The Output Driver 100 comprises an NMOS pull-up transistor 112 and an NMOS pull-down transistor 114. A problem with the Output Driver 100 of FIG. 1 is the NMOS pull-up transistor 112. NMOS transistors typically have a low Drive/Input Capacitance ratio. In order to provide sufficient drive capability, the NMOS transistor 112 must be relatively large, which increases the input capacitance of the driver. This forces the pre-amplifying stages 116 and 118 to provide more drive (current) in order to drive the Output Driver 100. The performance of the all NMOS Output Driver 100 is poor because it is characterized by a low Drive Capability/Input Capacitance ratio.
FIG. 2 shows a BiCMOS Output Driver 200. The BiCMOS Output Driver 200 comprises a bipolar pull-up transistor 204 and an NMOS pull-down transistor 206. The Output Driver 200 has better performance than the Output Driver 100 of FIG. 1 because of bipolar pull-up transistor 204. Bipolar transistors have better Drive/Input Capacitance ratios than do MOS transistors. A problem with Output Driver 200 is that it exhibits reliability problems due to tristating. When the Output Driver 200 is tristated, the base of bipolar transistor 204 is driven low by conducting tristate transistor 227, and the gate of transistor 206 is driven low by conducting tristate transistor 228, so that the output node Qout 202 is undriven by Output Driver 200. It is to be appreciated that in a typical system node Qout 202 is also coupled to Output Drivers of other semiconductor devices. When Qout is driven high by another semiconductor device while the base of bipolar transistor 204 is driven low, a reverse bias develops on the base/emitter junction of bipolar transistor 204. Such a reverse bias can degrade the electrical characteristics of transistor 204. The changing electrical characteristics of transistor 204 makes the Output Driver 200 unreliable.
Output Driver 300 of FIG. 3 eliminates the reliability problem associated with Output Driver 200 of FIG. 2. Output Driver 300 has a diode connected bipolar transistor in series between output node Qout 302 and bipolar pull-up transistor 304. Thus, when the base of bipolar transistor 304 is forced to ground during tristating, and Qout 302 is raised high by another semiconductor device, the reverse bias voltage is divided between the base emitter junctions of bipolar transistors 304 and 308. In this way each junction sustains only half the reverse bias which is not enough to degrade the electrical characteristics of transistor 304.
Unfortunately, however, the addition of bipolar transistor 308 causes the V.sub.OH level of Output Driver 300 to drop below specification requirements. The V.sub.OH level of an Output Driver is the voltage level outputted by the driver for a logical high input. For TTL level Output Drivers, V.sub.OH must be at least 2.4 volts. For Output Driver 300: V.sub.OH =V.sub.CC -V.sub.bus -V.sub.BE 304-V.sub.BE 308 (V.sub.bus is the voltage drop due to resistance on the power supply bus, typically 0.2 volts for large chips; V.sub.BE is determined from process parameters and typically varies from 0.6 volts at high temperatures to 1.0 volts at low temperatures). Under worst case operating conditions V.sub.CC =4.4 volts, V.sub.bus =0.2 volts, V.sub.BE 304=1 volt, and V.sub.BE 308=1 volt. Under such conditions V.sub.OH =2.2 volts for Output Driver 300, which is below the V.sub.OH minimum data sheet requirement of 2.4 volts. Thus, Output Driver 300 is unsuitable because under worst case operating conditions it does not meet minimum V.sub.OH specifications.
Output Driver 400 of FIG. 4 is an attempt to solve the V.sub.OH problem associated with Output Driver 300 of FIG. 3. Output Driver 400 includes an NMOS pull-up transistor 409 coupled in parallel with bipolar transistors 404 and 408. The NMOS transistor 409 provides a "boost" in the V.sub.OH level under worst case (low temperature) operating conditions so that Output Buffer 400 meets the minimum 2.4 volt V.sub.OH data sheet requirement. It is to be appreciated that MOS transistors exhibit their best electrical characteristics at cold temperatures due to better mobility. Thus, when temperatures are low and the V.sub.BE values of bipolar transistors 404 and 408 are the highest, the V.sub.OH level of Output Driver 400 meets minimum requirements because NMOS transistor 409 is strong at low temperatures and provides the necessary "boost" in the V.sub.OH level.
A problem with Output Driver 400 of FIG. 4 is that under high temperature conditions it exhibits poor performance. An Output Driver's performance can be represented by its Drive Capability/Input Capacitance. The larger the Drive/Input Capacitance ratio, the better the driver. At high temperatures MOS transistors have lower mobility, so their performance decreases. At high temperatures NMOS transistor 409 provides little assistance in pulling node Qout high, but yet still adds significant input capacitance to node 403. It is to be appreciated that the capacitive load added by NMOS transistor 409 is large because NMOS transistor 409 must be sized large enough to supply necessary current to meet the V.sub.OH limit during cold operating conditions. Because transistor 409 adds significant load without providing additional drive at high temperatures, the Output Driver 400 suffers a loss in performance at high temperatures.
Thus, what is desired is a high performance Output Driver capable of operating reliably and consistently over a wide temperature range.